1. Field of the Invention
The present invention relates to a drive circuit of an electromagnetic device for turning on and off an electromagnetic contactor or the like.
2. Description of Related Art
An electromagnetic contactor is turned on or off by energizing or deenergizing a coil wound on a steel core of the contactor, thereby attracting or releasing its armature. It is preferable that a drive circuit for such an electromagnetic device be small and inexpensive, and its attracting and releasing times be short. Thus, a conventional electromagnetic device is known whose coil is provided with a freewheeling circuit.
FIG. 1 shows a conventional drive circuit of an electromagnetic device for use with such an electromagnetic contactor. In FIG. 1, a coil 1, which is wound on a steel core not shown in this figure, is serially connected to the collector of a transistor 2. The base of the transistor 2 is connected to the output of a rectangular wave oscillator 3, and the emitter thereof is connected to one end of the oscillator 3. A serial circuit of a surge absorber 4 and a diode 5 is connected in parallel with the coil 1, and the surge absorber 4 is connected in parallel with the collector and emitter of a transistor 6. The serial circuit of the coil t and the transistor 2 is connected to a DC power supply 10 through a serial connection of diodes 71 and 72 and a switch 9. The base of the transistor 6 is connected to the connecting point of the diode 72 and the switch 9 through a resistor 73. The diodes 71 and 72, and the resistor 73 constitute a control circuit 7A of the transistor 6.
The operation of the drive circuit of the electromagnetic device is roughly divided into two operation modes. In a first operation mode, the switch 9 is opened immediately after the switch 9 and the contactor are closed. In a second operation mode, the switch is opened after the contactor is closed and then enters into a holding state.
FIGS. 2(A)-2(E) illustrate the operation in the first operation mode. The switch 9 is closed at time t.sub.0, and is opened again at timet.sub.1 immediately after time t.sub.0 as shown in FIG. 2(A). Closing the switch 9 at time t.sub.0 as shown in FIG. 2(A) impresses the voltage E.sub.1 of the power supply 10 across the series circuit of the coil 1 and the transistor 2, and the rectangular oscillator 3, through the two diodes 71 and 72. The oscillator 3 produces a starting pulse V.sub.p1 as shown in FIG. 2(B), and the coil 1 is energized by a starting coil current I.sub.11 as shown in (C) of FIG. 2(C). No current flows through the transistor 6 as shown in FIG. 2(D) because the diode 5 is reverse biased from time t.sub.0 to time t.sub.1, and the switch 9 is opened at time t.sub.1. Accordingly, the counter electromotive force generated by the coil 1 due to opening of the switch 9 at time t.sub.1 is impressed entirely across the serial circuit of the diode 5 and the surge absorber 4, and a rather large current i flows through the surge absorber 4 as shown in FIG. 2(E).
FIGS. 3(A)-3(E) illustrate the operation in the second operation mode. In this operation mode, the switch 9 is opened at time t.sub.4 after the contactor has entered into a holding mode. At time t.sub.0, the switch 9 is closed as shown in FIG. 3(A). Because switch 9 was closed, the oscillator 3 provides the base of the transistor 2 with a rectangular pulse voltage V.sub.p consisting of a wide width starting pulse V.sub.p1 and successive narrow width holding pulses V.sub.p2. Accordingly, a coil current I.sub.1 flows through the coil 1, thereby attracting the armature. In this case, a current due to the counter electromotive force of the coil 1 begins to flow through the transistor 6 at time t.sub.1 because the voltage drop across the two diodes 71 and 72 provides a forward bias to the transistor 6. Therefore, during intervals at which the pulse voltage V.sub.p is not applied to the transistor 2, the electromagnetic energy stored in the coil 1 flows through the transistor 6 as an electric current I.sub.2 as shown in FIGS. 3(D). Thus, the current I.sub.1 flowing through the coil 1 is the sum of the current supplied from the power supply 10 and the current I.sub.2.
When the switch 9 is opened at time t.sub.4, the voltage E.sub.1 of the power supply 10 and the output of the oscillator 3 are removed, and the transistor 6 is turned off. Thus, the current I.sub.2 is stopped. The electromagnetic energy stored in the coil 1 will flow as a current i through the diode 5 and the surge absorber 4. Since the surge absorber 4 has a large resistance, the current i rapidly attenuates, and the armature is released at time t.sub.6, thereby opening the electromagnetic contactor.
The conventional drive circuit of an electromagnetic device has a problem in the first operation mode, in which the switch 9 is closed at time t.sub.0, and opened again at time t.sub.1 immediately after the wide width starting pulse V.sub.p1 is applied from the oscillator 3 to the base of the transistor 2 as shown in FIGS. 2(A) and 2(B). In this case, a rather large current I.sub.1 flowing through the coil 1 (hereinafter referred to as a starting coil current I.sub.11) immediately changes its passage to the surge absorber 4, through which it flows as the current i. As a result, the surge absorber 4 is overheated, and may sometimes suffer damage.
To overcome this problem, another conventional drive circuit of an electromagnetic device as shown in FIG. 4 is proposed. The drive circuit of FIG. 4 has a capacitor 74 connected in parallel with the serial connected diodes 71 and 72 of the control circuit 7A of the drive circuit of FIG. 1. The diodes 71 and 72, the resistor 73 and the capacitor 74 constitute a new control circuit 7B.
The operation of the drive circuit of the electromagnetic of FIG. 4 device will be explained referring to FIGS. 5(A)-5(E) and 6(A)-6(E). FIG. 5 illustrates the first operation mode, in which the switch 9 is closed at time t.sub.0, and opened at timet.sub.1 immediately after the wide width starting pulse V.sub.p1 is supplied from the oscillator 3 to the base of the transistor 2, as in FIGS. 2(A)-2(E).
In FIGS. 5(A)-5(E), closing the switch 9 at time t.sub.0 induces a voltage drop across the diodes 71 and 72, which charges the capacitor 74 of the control circuit 7B. Then, opening the switch 9 at time t.sub.1 removes the voltage E.sub.1 of the power supply 10 and the output of the oscillator 3 from the coil 1 and the base of the transistor 2, respectively. The transistor 6, however, continues the on-state due to the electric discharge of the capacitor 74 during a delay time T until the discharge ends as shown in FIG. 5(D). During the discharge, the coil current I.sub.1 flows through the transistor 6 as the current I.sub.2. The current I.sub.2 gradually attenuates owing to the resistance of the transistor 6, and drops below the allowable current i.sub.0 of the surge absorber 4 at time t.sub.3 after the delay time T as shown in FIG. 5(D). At time t.sub.3, the transistor 6 is turned off, and the current i below the allowable current i.sub.0 flows through the surge absorber 4. The current i sharply decreases, the armature is released, and the electromagnetic contactor is opened. Thus, in the second conventional drive circuit of FIG. 4, since the current i flowing through the surge absorber 4 is limited below the allowable current i.sub.0, the surge absorber 4 is prevented from being overheated or burned.
The second conventional drive circuit, however, delays the off timing of the transistor 6 connected in parallel with the surge absorber 4. The delay is effective in the first operation mode because the rather large coil current I.sub.1 is reduced below the allowable current i.sub.0 of the surge absorber 4 during the delay time T as shown in FIGS. 5(C) and 5(D). The delay, however, serves only to delay the operation of the contactor in the second operation mode.
FIGS. 6(A)-6(E) illustrate the second operation mode of the second conventional drive circuit as shown in FIG. 4. In the second operation mode, the switch 9 is opened after the contactor has entered into a holding mode, in which the contactor is held by holding pulses V.sub.p2 outputted after the starting pulse V.sub.p1 as shown in FIGS. 6(A)-6(C). The transistor 6 continues the onstate for the delay time T after the switch 9 is opened at time t.sub.4. This serves only to delay the opening operation of the contactor because it is unnecessary to reduce the small holding current of the coil 1 in this case.
Since the frequency of the second operation mode is much higher than that of the first operation mode, the delay involved in the second operation mode is undesirable.